Line or lines in eyebrow is one, two or maybe more thin shaves into either left right or either both eyebrows, it can represent your a part of a gang culture like bloods and crips which is where is came from or what most people do is just ask their barber or hairstylist to do it just for a style.

Can I have a line in eyebrow please, then you say how many in which one or how many in both, this can be done at a barbers/hairsylist.